The present invention relates to a system which purifies or controls exhaust emission from an internal combustion engine of a vehicle and a method of purifying it. Particularly, it relates to an exhaust gas-purification system used for a vehicle or a hybrid type electric vehicle having an internal combustion engine which can be operated in a lean air-fuel ratio (lean burn) and a method of purifying the exhaust gas.
Carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), etc., contained in exhaust gas exhausted from internal combustion engines used for vehicles are well known as air pollutants.
Great deals of efforts have been made for reducing release of these substances. Improvements in the combustion method of the internal combustion engine have fundamentally reduced the generation amounts of the pollutants. Further, there have been pursued developments of a method for purifying the exhaust gas by utilizing a catalyst or the like. These efforts yield steady results.
In particular, with regard to a gasoline engine vehicle, the mainstream is a method of utilizing a catalyst containing Pt and Rh as main active components, which is a three-way catalyst and performs oxidation of HC and CO and reduction of NOx at the same time.
By the way, the three-way catalyst generally has physical properties in which the three-way catalyst effectively purifies only exhaust gas generated by burning gasoline fuel nearly at the stoichiometric air-fuel ratio.
While the air-fuel ratio varies depending upon the operation condition of vehicle, the range of variation thereof is adjusted nearly at the stoichiometric air-fuel ratio in principle. In the case of gasoline fuel, weight of air (A)/weight of fuel (F)=about 14.7; the stoichiometric air-fuel ratio will be represented by A/F=14.7 in the present specification hereinafter, while the above value may slightly varies depending upon the kind of fuel.
When the internal combustion engine is operated in a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, fuel economy can be improved. Therefore, there is also proposed a technique for performing combustion with leaner air-fuel mixture in view of resource saving.
Under the above circumstances, developments of a lean-burn combustion technique have been pursued. Recently, it becomes possible to burn an air-fuel mixture in a lean range of an air-fuel ratio of 18 or higher.
However, when a lean-burn exhaust gas is purified with a conventional three-way catalyst as described above, the problem is that HC and CO can be oxidized but NOx can not be effectively reduced.
Therefore, for applying a lean-burn system to heavy duty vehicles and extending engine operating area in the lean-burn combustion, in other words, extending an operation range to which the lean-burn system may be applied, there is required a lean-burn compatible technique of purifying the exhaust gas.
So, there have been vigorously pursued developments of the lean-burn compatible technique for purifying the exhaust gas, that is, a technique which purifies HC, CO and NOx in exhaust gas containing a considerable amount of oxygen (O2), particularly developments of a technique which purifies NOx.
These techniques are proposed in, for example, publications of WO93/07363 and WO93/08383, JP-A-9-317447 publication and JP-A-11-62653 publication.
According to the techniques disclosed in the publications of WO93/07363 and WO93/08383, the exhaust gas always flows in a NOx absorbent, NOx is absorbed when the air/fuel ratio is lean, and when the concentration of O2 in the exhaust gas is decreased, the absorbed NOx is released, whereby the absorbent may be reactivated.
However, a material which can absorb NOx when the air/fuel ratio is lean, and release NOx when the concentration of O2 in the exhaust gas is decreased, has the following problems. The above material absorbs sulfur contained in the exhaust gas so that its NOx absorption capability is decreased. Further, when the concentration of O2 in the exhaust gas is decreased, the fuel economy of the internal combustion engine deteriorates so that the improvement in the fuel economy owing to the lean-burn is impaired.
Further, the JP-A-9-317447 publication discloses a technique which estimates the sulfur absorption amount of a NOx catalyst and performs a treatment for decreasing the concentration of O2 in the exhaust gas when the estimated value exceeds a predetermined amount. However, the problem is that the fuel economy of the internal combustion engine deteriorates.
Furthermore, the JP-A-11-62653 publication discloses a technique for absorbing a torque shock of a hybrid vehicle having a lean NOx catalyst at a NOx reactivating treatment by means of regeneration braking of a motor. However, similarly to the techniques disclosed in the above-mentioned publications of WO93/07363 and WO93/08383, the problem is that the NOx adsorption catalyst deteriorates due to the sulfur contained in the exhaust gas.
It is an object of the present invention to prevent variation of an increase in the torque or output of an internal combustion engine(these are generically called xe2x80x9cengine outputxe2x80x9d hereinafter, and, of course, the same definition is used throughout the claims) at a time of reactivating a NOx catalyst, and concurrently recover an energy corresponding to the increased proportion of the engine output.
According to the present invention, there is provided an exhaust gas purification method of an internal combustion engine. The internal combustion engine comprises a generator driven by said internal combustion engine, a battery charged with an electricity generation output from the generator, and a NOx adsorption catalyst placed in an exhaust gas passage. The catalyst chemisorbs NOx in a state where the amount of an oxidizer is larger than that of a reducer in the stoichiometry relation of oxidoreduction between components contained in exhaust gas exhausted from said internal combustion engine and catalytically reduces the adsorbed or absorbed NOx in a state where the amount of the reducer is at least equivalent to that of the oxidizer. When the time-elapsing deterioration of the NOx adsorption catalyst reaches to a predetermined degree, the combustion condition of said internal combustion engine is controlled at the stoichiometric air-fuel ratio or an excess fuel air-fuel ratio in a predetermined time period to reactivate the NOx adsorption catalyst from the time-elapsing deterioration, and the increased proportion of the engine output generated during the above time period is absorbed with said generator.
According to the present invention, further, there is provided an exhaust gas purification system of an internal combustion engine. The internal combustion engine comprises a generator driven by said internal combustion engine, a battery charged with an electricity generation output from said generator, and a NOx adsorption catalyst placed in an exhaust gas passage. The catalyst chemisorbs NOx in a state where the amount of an oxidizer is larger than that of a reducer in the stoichiometry relation of oxidoreduction between components contained in exhaust gas exhausted from said internal combustion engine and catalytically reduces the adsorbed or absorbed NOx in a state where the amount of the reducer is at least equivalent to that of the oxidizer. The engine further comprises a time-elapsing deterioration evaluation means for evaluating that the time-elapsing deterioration of the NOx adsorption catalyst reaches to a predetermined degree, an air-fuel mixture control means for controlling the combustion condition of said internal combustion engine to the stoichiometric air-fuel ratio or an excess fuel air-fuel ratio in a predetermined time period when it is evaluated by said time-elapsing deterioration evaluation means that the time-elapsing deterioration of the NOx adsorption catalyst reaches to a predetermined degree, and an absorption means which makes said generator absorb the increased proportion of the engine output generated while the combustion condition of the internal combustion engine is controlled to the stoichiometric air-fuel ratio or an excess fuel air-fuel ratio by said air-fuel mixture control means.